The allowable ampacity for 4/0 AWG copper wire is 195 amps in the 60°C column, 230 amps in the 75°C column, and 260 amps in the 90°C column, per NEC Table 310.16. However, the number you can actually use to size your breaker depends entirely on your termination ratings, insulation type, and installation environment. Pulling 260 amps through a 4/0 conductor into a standard residential panel will likely melt the lugs if you ignore the termination temperature rules.

NEC Table 310.16: 4/0 AWG Copper Ampacity Chart

This reference table is extracted from the NFPA 70 National Electrical Code (Table 310.16, formerly 310.15(B)(16)). It applies to copper conductors with up to three current-carrying conductors in a raceway or cable, at an ambient temperature of 30°C (86°F).

How to read this table: The columns represent the temperature rating of the wire's insulation, not the temperature of the room. Common 90°C insulations include THHN, THWN-2, and XHHW-2. Common 75°C insulations include THW and THHW. The 60°C column applies to older insulations like TW, as well as modern NM-B (Romex) cables, which are legally capped at 60°C regardless of the 90°C rating of the individual wires inside them.
Table 310.16: Allowable Ampacities of Insulated Copper Conductors (60°C to 90°C)
AWG / kcmil 60°C (140°F) Column 75°C (167°F) Column 90°C (194°F) Column
2/0 AWG 145A 175A 195A
3/0 AWG 165A 200A 225A
4/0 AWG (Target) 195A 230A 260A
250 kcmil 215A 255A 290A

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is looking at the 90°C column for THHN wire and assuming they can put a 250A breaker on a 4/0 copper feeder. You cannot. The ampacity of the entire circuit is limited by the lowest temperature rating of any connected component, a rule enforced by NEC 110.14(C).

Here is how to determine your actual limit:

  • The 75°C Column (230A): This is the default for almost all modern commercial and residential feeder installations. Breakers, panelboard busbars, and mechanical lugs rated 100A and above are almost universally tested and listed at 75°C. If you are landing 4/0 THHN in a 200A or 225A main breaker, your maximum allowable ampacity is 230A.
  • The 60°C Column (195A): You must use this column if you are using NM-B (Romex) cable, if the equipment is explicitly marked for 60°C only, or if you are connecting to older, unmarked equipment rated 100A or less. Even if you use 90°C XHHW-2 wire, if it transitions into a 60°C-rated disconnect switch, the entire circuit drops to 195A.
  • The 90°C Column (260A): You are almost never allowed to use this column for final breaker sizing. The 90°C column exists primarily to give you a higher baseline number for calculating derating factors (explained below).

Derating 4/0 Copper: Ambient Heat and Conductor Bundling

The base ampacities in the chart above assume you have no more than three current-carrying conductors in a conduit and the ambient air temperature is 86°F (30°C). When conditions change, the wire cannot shed heat as efficiently, and you must reduce (derate) the allowable current. Derating calculations always start from the 90°C column.

Scenario A: High Ambient Temperature
You are running a 4/0 THHN feeder through an unventilated attic in the summer where the ambient temperature reaches 113°F (45°C). According to the NEC ambient temperature correction factors, the multiplier for 41°C–45°C at 90°C insulation is 0.82.
Calculation: 260A (90°C base) × 0.82 = 213.2A.
You then compare this derated value (213.2A) to your termination limit (usually 230A for 75°C). The final allowable ampacity is the lower of the two: 213.2A.

Scenario B: Conductor Bundling
You are pulling four current-carrying conductors (e.g., two hots, a neutral carrying unbalanced load, and a grounded phase for a multi-wire branch circuit) through the same PVC conduit. NEC Chapter 9 requires an 80% adjustment factor for 4-6 current-carrying conductors.
Calculation: 260A (90°C base) × 0.80 = 208A.
Again, comparing 208A to the 75°C termination limit of 230A, your new legal maximum is 208A. You would need to drop your breaker size to 200A.

Pro-Tip on Grounding Conductors: When counting current-carrying conductors for derating, equipment grounding conductors (EGCs) do not count. However, if you are running a neutral that carries only unbalanced load from a standard single-phase 120/240V residential service, the neutral does not count as a current-carrying conductor for derating purposes per NEC 310.15(B)(5). Always verify the load type before counting.

What the Ampacity Table Cannot Tell You

While the Copper Development Association and the NEC provide excellent thermal limits for the wire itself, ampacity tables do not account for the physical realities of the jobsite. Before ordering 500 feet of 4/0 copper, check these three factors:

1. Voltage Drop Over Distance
Ampacity only tells you the wire won't melt; it doesn't guarantee your equipment will get enough voltage. NEC 310.15(B) recommends a maximum 3% voltage drop on feeders. If you are pulling 200 amps through 4/0 copper over a distance of 250 feet to a detached workshop subpanel, you will experience roughly a 4.8% voltage drop. To maintain the 3% threshold at that distance and load, you would need to step up to 250 kcmil or 300 kcmil copper, despite 4/0 having the thermal ampacity to handle the heat.

2. Physical Lug Fitment
4/0 AWG copper is roughly 0.46 inches in diameter without insulation, and over half an inch with THHN insulation. Many standard 200A residential main breakers and older meter sockets have lugs physically designed to max out at 2/0 or 3/0 AWG. If you cannot physically seat the 4/0 wire into the lug without shaving strands (a massive code violation and fire hazard), you will need to use a listed mechanical reducing pin or a lug adapter kit approved by the panel manufacturer.

3. Bending Radius and Conduit FillNavigating 4/0 wire through tight junction boxes is notoriously difficult. NEC 300.34 mandates strict minimum bending radii for conductors to prevent damaging the insulation and the copper stranding. For shielded or lead-covered wire, the radius is 12 times the diameter, but for standard unshielded 4/0 THHN, you must allow a bending space of at least 8 times the overall diameter of the wire. Furthermore, 4/0 wire takes up massive cross-sectional area; three 4/0 THHN conductors plus a #4 copper ground will completely max out a 2-inch PVC conduit (40% fill limit). You will likely need to pull 2.5-inch or 3-inch conduit to avoid jamming the wires during the pull.